I had to switch off Time Machine on MacbookHD (main disk) by excluding it from the Time Machine preferences, and reinstall the OS to erase the previous copies it had made, all ~400GB of it (on my 499GB disk)... As the user, I was not asked for permission to allow Time Machine to use up to 80% of available disk capacity. It was backing up so many copies of the data until I noticed I had left only 8GB of free space and I only noticed because Quicktime screen recording started crashing due to insufficient disk space. Basically, the decision by Apple is to use up to 80% of the available disk space to save copies of the data on that disk without user permission, and because it checks and reconciles the amount it's using every so often you can easily end up with 0% free capacity if you e.g. record a lot of videos in between.